As airplane avionic systems have become more and more sophisticated, an increased need has developed to provide line maintenance technicians with maintenance assistance features. In response to this need, airline companies have required airplane manufacturers to provide built-in test equipment (BITE) for the various onboard avionic systems. The BITE systems provide diagnostic information in the case of system malfunctions, and related maintenance features.
Avionic systems are comprised of numerous line replaceable units (LRUs), each of which constitutes a replaceable electronics component. The early attempts at providing BITE within analog avionic systems were not particularly successful, and typically resulted in the use of shotgun maintenance techniques wherein many LRUs or entire systems were replaced until a fault was corrected. With the advent of digital avionics, the opportunity arose to provide sophisticated and effective BITE systems. In a modern digital avionic system, each LRU communicates with other LRUs primarily by means of serially transmitting and receiving digital information formatted in standarized multi-bit words. For example, an LRU may sense the altitude of the aircraft and then formulate a digital word that, among other things, includes a binary representation of the altitude. This digital word is transmitted to another LRU which, for example, might be a digital display system on the instrument panel in the cockpit. The display unit decodes the digital word and displays the altitude in an appropriate decimal format. Other system subgroupings may consist of analog sensors that communicate via discrete and dedicated analog signal lines.
While the LRU altimeter is but one example of a line replaceable subsystem, it will be appreciated that the actual avionic systems in large sophisticated aircraft involve many diverse LRUs which are interconnected by serial buses to form a complex intelligence system. Because of the complexities and scale of such avionic systems, the aircraft industry has taken steps to standardize the digital words that are transmitted and received by the various LRUs so as to facilitate the interchangeability and maintenance of such components.
One aspect of this standardization is that a large portion of the information transmitted and/or received by the LRUs is in the form of standardized digital communication words, wherein each word includes a data field, a label, and other ancillary information. An example of such a standardized digital word is specified in a publication prepared by Aeronautical Radio, Inc. of Annapolis, Md., entitled "Mark 33 Digital Information Transfer System (DITS) Specification No. 429-4", published August 1980, the contents of which are incorporated herein by reference. The communication word specified in this specification is known in the industry as the ARINC 429 word. The ARINC 429 word is comprised of 32 bits in a standardized format which can be used to represent over 300 different functions or parameters generated and/or received by commonly employed LRUs of an avionic system. The information carried by the various 32 bit communication words flows from one LRU to one or more other LRUs over a bus comprising a twisted and preferably shielded pair of wires in which only unidirectional serial flow of the digital word or words is permitted. Thus, when bidirectional communication is required between two LRUs, they will be coupled by at least two separate buses.
Previously, to provide BITE systems in digital avionic systems, a centralized BITE scheme was used wherein fault information detected by an LRU was encoded by the LRU and transmitted to a centralized computer unit. This centralized computer unit provided all of the means for displaying data and interacting with a maintenance technician, and included means for converting the binary codes generated and transmitted by the LRUs into English language messages. Because in these systems the LRU that conducts the test or detects the failure does not actually present the fault data, the systems are not user oriented or "friendly". In such systems, the binary codes transmitted from the testing LRU to the display unit have to cover complex subsystems, such as an autopilot, as well as simpler subsystems, such as entertainment subsystems. The requirement for a single BITE system to cover such diverse subsystems tends to make the BITE system too rigid and limits its effectiveness.